Generally, the phrase “cranial stabilization” products or devices refers to a line of compatible and interconnectable medical devices used during neurosurgery to hold the head of a patient in a fixed position relative to a surgical operating table. A typical arrangement of such products may include, for example, a base unit which connects directly to the surgical table, one or more adaptors connected to the base unit, and a skull clamp connected to the adaptor the skull clamp having three skull pins which engage and hold the skull of the patient. The present assignee has commercialized a successive number of three pin skull clamps under the MAYFIELD® trademark. One particular skull clamp which has enjoyed significant market success for a long period of time is the three pin skull clamp shown and described in U.S. Pat. No. 4,169,478, entitled “Surgical Skull Clamp” which is expressly incorporated herein by reference in its entirety. The present invention relates to a three pin skull clamp similar to the one shown in the '478 patent.
As shown and described in the '478 patent, a three pin skull clamp includes a C-shaped frame defined by two legs which connect to each other via an adjustable ratchet mechanism, which adjusts to vary the space between the ends of the legs. The frame partially encircles the head of the patient, with the opposite ends of the legs residing on opposite sides of the head, in alignment with an imaginary axis through the head.
At an end of a first of the two legs of the C-shaped frame, the skull clamp has a single pin assembly, specially a threaded pin carrier and a single skull pin, oriented along the axis. This single pin assembly threadably connects to the end of the first leg, in alignment with the axis, and is threadably movable along the axis relative to the frame. This structure enables a surgeon to threadably move the single skull pin toward or away from the head of the patient. Preferably, the assembly is spring-loaded to provide some “give” for the single skull pin aligned along the axis and to enable measurement of the force applied by the single pin to the head of the patient. Generally, this force is in the range of about sixty to eighty pounds.
At the end of the other leg of the frame, on the opposite side of the head, two skull pins engage the head. These two skull pins are mounted in spaced relation to a rocker arm held at the end of the second leg. Preferably, the rocker arm is selectively rotatable about the axis, along with a swivel adaptor when not in a locked mode. When locked, the swivel adadtor is fixed relative to the frame and with respect to the axis. The mechanism for permitting this selective locking and unlocking, and consequently the nonrotation and rotation of the rocker arm and swivel adaptor about the axis relative to the frame, respectively, is disclosed and claimed in the aforementioned '478 patent. This mechanism forms no part of the present invention.
In addition to rotating about the axis with the swivel adaptor, the rocker arm is also pivotal about a pivot point relative to the swivel adaptor, to vary the respective distances between the rocker arm skull pins and the axis. Together the rotation of the rocker arm and the swivel adaptor, and the pivoting of the rocker arm relative to the swivel adaptor provide versatility in securing the three point skull clamp to the head, in a manner which assures consistently secure engagement of the skull in a fixed position during surgery.
Regardless of the adjustability of the rocker arm, with any three pin skull clamp the engagement forces at the rocker arm skull pins must be distributed equally, or at least close to equal. Otherwise, there is a possibility that the head of the patient will slip from engagement with the pins of the skull clamp, possibly resulting in serious injury to the patient. In many cases the single pin side is threadably moved so as to engage the skull of the patient with a desired amount of axial engagement force, and the head is securely held at the opposite end of the clamp by the two spaced pins mounted to the rocker arm. If the rocker arm is properly aligned on the head of the patient, the two rocker arm skull pins will be oriented at equal opposing angles relative to axis of the single pin, and the axial engagement load will be distributed evenly between the two rocker arm pins. For instance, if the single pin skull clamp is adjusted to achieve eighty pounds of force on the skull, and if the head is properly secured, there will be equal load distribution of the engagement force between the two rocker arm skull pins. Namely, this force for each of the rocker arm pins should be forty pounds multiplied by the cosine of the angle of the rocker arm pins relative to the single pin axis. This represents the normal load on each of the two rocker arm pins.
If the rocker arm is not properly aligned with the axis of the single pin, the normal load components on the two rocker arm pins will be unequal. Although it is not absolutely necessary for these engagement forces to be exactly equal, the stable holding of the head requires that those engagement forces be reasonably close to equal. Up to now assignee has not known of any practical way to determine with reasonable certainty whether there is an equal load distribution of the engagement forces at the rocker arm skull pins.
Although it has been fairly common to use the threaded pin assembly to measure the engagement force on the spring-loaded single pin at one end of the skull clamp, that same practice is neither practical nor desirable at the rocker arm end of the skull clamp. For one thing, mounting of a pair of similarly constructed threaded single pin assemblies to the opposite ends of the rocker arm would likely interfere with free rotation of the rocker arm and the swivel adaptor about the axis, when in the unlocked mode.
Also, the single pin assembly threadably adjusts relative to the first leg of the frame to enable the movement of a single pin to eventually cause head engagement by all three pins. It is neither desirable nor practical for the neurosurgeon or surgery room attendants to threadably adjust three separate skull pins in order to engage the head of the patient. Also, it is desirable to have the two spaced rocker arm skull pins at about the same distances from the axis, to facilitate the obtaining of equal distribution of the load. But if these two pins were separately adjustable it would be much more difficult to consistently determine if equal load distribution has been achieved.
It is therefore an object of the present invention to make it easier for surgeons and surgery, room attendants to determine that there is a sufficiently uniform load distribution on the two spaced skull pins at the rocker arm side of a three pin skull clamp.
It is another object to the present invention to facilitate the quick and easy detection of unequal load distribution between the two spaced skull pins mounted to the rocker arm of a three pin skull clamp.
It is still another object of the present invention to achieve these aforementioned benefits in a simple manner, and within the framework and structure of a well known and highly successful skull clamp of the type shown and described in the '478 patent.